Cavity wall insulation? yes no maybe?

Seems like it is second on the list after loft insulation, but seem to remember a few horror stories from the past, moving in to an early (c1983) house at the end of the month & wondered if it would be a good idea or not?

This is a very subjective subject. A new build, detailed correctly should not pose any issues for the future, but retro-fitting to an older building has potential problems.

Cavities were introduced for two main reasons, insulation and ventilation.

Yes, the insulative quality of a void is not as good as filling it with specific material, but in doing so you can reduce if not sometimes eliminate the ventilation of moisture.

This can cause problems inside the internal leaf, but the biggest problem is if the two leaves have wall ties. New stainless wall ties with a drip are ok, but cavity insulation which holds moisture against older ties will soon corrode them, specifically if the insulation has a high Sodium Silicate & Chloride content such as a Rockwool type product.

I have been involved (years ago now mind) where the external leaf has been removed to re-fit wall ties and then re-build due to cavity insulation corroding the ties. There are new retro-fit methods that do not require the removal of the outer leaf, but can still be costly.

I'm not against it personally, but ask the installer plenty of questions and ensure he does the job correctly for your wall construction.

Ask lots of questions. We had our 1970s detached done 2-3 years ago. House is warmer, but condensation has got really bad, and we have issues with mould in some bedrooms where the external wall meets the ceiling, which a builder friend thinks is where the insulation isn't right to the top of the wall creating cold spots. Also seems to be a damp problem starting low down on one wall.

Not sure we'd do it again. Solved one problem but created others, but other people may have had better eexperiences.

Main issues as pointed out above are insulations voids causing cold spots, condensation and eventual mould, water penetration (cavity walls originated on the West coast of the UK as a wall of stopping water penetrating the wall, insulating it was a later idea.

It's easy to get in, not so easy to get out if it's bodged. Most installations are fine, when it goes wrong it's a major issue. Lots of dodgy contractors out there making easy money as well.

Given the property you are moving into it may have already been done, if not it's not likely to have significant insulation in there. My house was built in 2001 and had to have thermolite internal blockwork to give the appropriate U value. Later on in the estate they changed the brick to a less porous one and used slab insulation in the cavity attached to the inner blockwork. This sort of insulation still gives a cavity for the water to run down.

Cavities were introduced for two main reasons, insulation and ventilation

No they were not.
They were introduced post war as we had crap, porus bricks and building materials. So the new houses were sucking up and letting in water. Building two walls was so that the outer 'sucked up' the moisture and sheltered the inner wall.
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We then have to come up with all sorts of iffy ways of filling cavity for warmth, windows that cannot sit in reveals (so shortening the life of them and contributing to overheating), iffy drainage and structural details, complex to build (and regularly get wrong) etc etc. However, this means that we now have a building system, trades and attitude built around building cavities, and know little else.
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The *last* thing you want for good insulation is a drafty gap in your wall. It is an awful way of building, and UK is in the minority to use cavities.
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/end rant

Gleen that opinion direct from Wikipedia? Yes, bricks are porous but once inside the cavity, how does this moisture get out? Via the found?

It is for the exact same reason your eaves are ventilated.

There are many reasons why the cavity was introduced, not just shody materials. How often do you see masonry piers in modern houses? It's s benefit of the additional depth the cavity brings when tied correctly.

The cavity wall method of construction was introduced in Northwest Europe during the 19th century and gained widespread use from the 1920s. In some early examples stones were used to tie the two leaves of the cavity wall together.[1] Initially cavity widths were extremely narrow and were primarily implemented to prevent the passage of moisture into the interior of the building.

From Wikipiedia

Much of the moisture or water will either come out the weep holes above lintels etc or run down the inside of the cavity to the ground which is below the level of the internal damp proof course.

Talking of moisture can be a little misleading, when I first moved into my house I was in the single skin garage one night with the wind driving the rain against the outside. So much water was coming through the bricks I could see it running down the inside of the walls which explained why all my tools had rusted

As alluded to further up, it no doubt depends on the house and location. Our 1950s house in the west Highlands had cavity insulation put in by the previous owner - probably for nowt as she was old and lots in the street were done at the same time.

We had it removed last year due to the damp problems it was causing, at a cost of £4500. Damp all gone since it's removal. The guys who extracted it drilled some big holes at the bottom to suck it out, and re drilled the installation holes to blow it down from top to bottom. They found at the not-overlooked rear of the house that some of the higher up installation holes didn't even go through the brick - the cowboys who put it in had drilled in about half an inch then put the plugging the hole back up stuff in the hole, just to make it look like it had been done, so that wall wasn't even filled properly!

MrsP is a structural engineer and when we were looking at it she checked with a lot of her colleagues who (should) know about this sort of thing. Having said that they're also a naturally risk averse lot due to the nature of the work they do. Anyhow, the upshot was that she decided that, for us, it wasn't worth the potential hassle with regard to damp, condensation etc and the possible cost of extraction would outweigh any short term savings. There again our neighbours had it done for nowt and they don't seem to be suffering...

The walls are tied together with big lumps of stone, so no installer was prepared to guarantee being able to fill the bits directly underneath each tie, which will lead to cold spots/condensation/damp.

Pretty sheltered house in the sunny south, so can't imagine there would be too much driving rain, will ask around in the rest of the close to see if anyone else has done it or not - prob leave it until I replace the windows sometime next year - got a rather large garage to sort out first